Plastic films comprising polyolefinic polymers, e.g. of oriented polypropylene (OPP) are widely used in food and non-food packaging and containers because of their desirable properties, such as resistance to the transmission of moisture, air, deleterious flavours and the like, as well as their excellent mechanical properties. However, a disadvantage of many of these polyolefinic films is the difficulty they have in readily forming seals at low temperatures either between the same surface of the film (denoted A to A sealability) as well as with the reverse side (denoted A to B sealability).
Prior art sealable BOPP films have been coated with PVdC (poly vinylidene chloride) coatings on one or both sides which provides a sealable coating as well as a high barrier to gases (such as oxygen). Such PVdC coated films are currently used mainly in the salad and fresh produce packaging markets but also for confectionery and biscuit packaging. However there is a market preference for non PVdC materials on environmental grounds. It is therefore desired to find an A/B sealable film with good anti-mist properties and a wide sealing range to replace PVdC films and which is preferably chlorine-free. A replacement product should also have robust sealability under wet and contaminated conditions.
It has been proposed to overcoat one or both surfaces of a film with a low temperature sealable coating (LTSC) which will readily A to A seal as well A to B seal after optional suitable treatment of the reverse surface (B side) and which is chlorine free.
U.S. Pat. No. 5,451,460 (Mobil) describes a heat sealable anti-fog coating composition. U.S. Pat. No. 5,419,960 (Mobil) discloses a base polymer film coated with a low temperature sealable coating (LTSC) comprising a copolymer of about 65% to 95% ethylene and about 5 to 35% of acrylic or methacrylic acid based on the weight of polymer, where about 2 to about 80% of the carboxylate groups are neutralised with metal ions from groups Ia, IIa or IIb of the Periodic Table.
U.S. Pat. No. 6,013,353 discloses a multilayer film comprising a film substrate having a polypropylene core layer (OPP) with skin layers on each side of the core comprising polymers of lower melting temperature than the core. The film is metallised on at least one side and coated therein with a polymeric, low temperature sealable coating (LTSC) comprising a copolymer of about 10 to 35 wt. % of at least one α, β-ethylenically unsaturated carboxylic acid, e.g., acrylic or methacrylic acid, with about 65 to 90 wt. % of ethylene, an alkyl acrylate or methacrylate, acrylonitrile, or mixtures thereof. The LTSC is stated to protect the metal coat and provide a film with improved A to B sealability.
WO 00/63309 (Du Pont) discloses a low temperature sealing compositions of EAA copolymers which have a low melt flow index (MFI) and are therefore easier to process.
However films with a LTSC also have a tendency to block (i.e. adhere together) as well as seal. Therefore it would be desirable to provide films with a LTSC to readily provide improved sealability but which also have a reduced tendency to block on the reel.
Prior art films with a LTSC also tend to exhibit poor anti-mist performance than those without a LTSC so it would be desirable to provide improved LTSC films which provide satisfactory or improved anti-mist performance to prior art LTSC films.
Thus it is an object of the invention to solve some or all or the problems described herein with prior art films coated with known LTSCs. It is a preferred object of the invention to provide films particularly suitable for packaging applications which have an good (more preferably optimum) balance between properties such as sealability at low temperatures (especially A to B), reduced tendency to block and satisfactory anti-mist performance.
Therefore broadly in accordance with the present invention there is provided a self supporting film of polyolefinic polymer(s), the film having a first and a second surface where a first exposed surface of the film (surface ‘A’) comprises a polymeric low temperature sealable coating (LTSC) and where the second exposed surface of the film (reverse side surface ‘B’) comprises an olefinic polymer capable of forming a seal to the LTSC at a minimum temperature of at most about 100° C. when measured at 15 psi for 2 seconds, such that the first and second surfaces of the film have a mutual heat seal strength (A to B) of at least about 200 g/25 mm, when measured at 15 psi for 2 seconds at 120° C.; where the LTSC also comprises:    (a) an anti-block additive in from an effective amount to about 15% by dry weight of the coating to give a static coefficient of friction of less than about 0.9 and a dynamic coefficient of friction less than about 0.75, such the first and second surfaces have a mutual blocking strength (A to B) of less than about 25 g/25 mm measured at 40° C. and 0.5 psi for one hour;    (b) a non-ionic anti-mist additive having an HLB value of from about 10 to about 20 in an effective amount to about 15% by dry weight of the coating; such that the film exhibits a substantially clear appearance on exposure to a relative humidity of at least 30% at 5° C. after one hour; and    (c) optionally an anti-slip additive of a mean particle size of from about 3 to about 5 microns in an effective amount up to about 0.5% by dry weight of the coating